Polymer matrix lithium ion cells are the newest of the commercially available battery cells. During the production of these cells, an electrolyte salt solution (electrolyte salt and electrolyte solvent) is imbibed into a polymeric matrix structure to yield a functional, "activated" battery system. Because the battery structure is defined by a polymer matrix, the electrolyte salt solution permeates the entire battery structure. Liquid electrolyte also commonly extends beyond the borders of the battery, filling or partially filling the battery packaging. It is common to use a heat-sealable laminate as the packaging material.
Lithium ion battery packaging may be plastically formed, or may be pouched. Both formed and pouched packaging structures are generally heat-sealed. A pouched structure is shown in FIG. 1. A laminated polymer matrix battery structure 110 is enclosed between heat-sealed laminated layers 112a, b which form a pouch. The pouch is sealed around electrode tabs 114 while permitting the tabs 114 to extend from the otherwise-enclosed pouch. This permits electrical contact to, and use of, the battery via the electrode tabs 114. As noted previously, electrolyte 116 may also be found within the pouch.
Battery packaging materials will vary with the battery manufacturer and the specific application. However, a typical packaging laminate, 212a, b, of the prior art is shown in FIG. 2. This represents a laminate layer 212a, b on each side of a metal foil layer 220, such as that taken at line a--a of FIG. 1. This shows two layers of laminate, 112a and 112b, which have been heat-sealed to enclose the battery pouch while permitting the electrode tab 214 to extend past the borders of the pouch.
The laminate material 212a, b includes a foil layer 220. The foil layer is located generally centrally within the laminate. At one surface of the foil layer is the inner laminar region 213. At the opposite surface of the foil layer is an outer laminar strata 222. The outer laminar strata 222 is provided as a package electrical insulator, for physical package protection, and for package marking or labeling.
The foil layer 220 acts as a barrier to oxygen and moisture incursion into the battery package. The foil also provides rigidity to the package, and provides structural support if the package is pre-formed into a specific shape. However, metal foil 220 can be reactive with lithium ions (Li.sup.+) of the electrolyte. An inner strata 213, designed to be nonreactive with the electrolyte, is provided between the foil layer and the package. The inner laminar region 213 should be inert to the enclosed electrolyte salt solution during battery storage and during battery use.
An inner laminar region 213 is positioned between the metal foil and the inside of the package. The inner laminar region has several functions. It provides a physical barrier between the metal foil and the electrolyte salt solution. Upon sealing, it provides a nonconductive barrier between the metal foil and the electrode tabs, which preliminarily prevents package shorting. It aids in plastic formation of the package, and it provides a self-sealing bond. Two layers of the package laminate can be placed with the inner laminar layers abutting each other. When an appropriate heat and pressure are applied, a heat-seal bond is produced.
The inner laminar region 213 generally includes a layer of polyamide 224 such as a nylon, and a heat-fusible sealing layer 226. Other layers which may be present but which are not pictured include tie layers (which act to adhere layers in position), or layers which promote formation or duration of the laminate material or of the pouch.
The polyamide layer 224 is located between the metal foil layer 220 and the heat-sealable layer 226. The polyamide layer acts as an insulative layer, keeping the body of the battery electrode tab electrically isolated from the body of the metal foil layer. This separation of the electrode tab and the package foil layers avoids electrical shorts between the body of the battery tab(s) and the body of the laminar package.